Analog and digital correlators herein described are suitable for use in signal processing applications such as sonar, radar, communication, frequency domain beamforming, and image transform apparatus.
The cross-correlator or cross-convolver is a powerful signal processing module for many sonar, radar, communication, beamforming, and image processing tasks which require linear filtering, convolution, cross-correlation, or Fourier transform calculation. For such tasks the cross-correlator has a high degree of computational parallelism and flexibility, with minimal control overhead.
The primary limitation of present analog cross-correlators is the attainable accuracy and the fact that a hybrid technology is generally required for their implementation. Representative examples are the surface acoustic wave (SAW) plate convolver and the SAW diode convolver. The plate convolver depends upon the nonlinear interaction between a pair of SAWs on a common piezoelectric substrate. Since the interaction is very weak, the device suffers from high insertion loss. The SAW diode correlator uses taps which are electrically coupled to diodes on a second substrate to provide the required mixing or multiplication. This presently requires a hybrid fabrication procedure and hence many separate electrical bonds, increasing the time required for fabrication and limiting the reliability of the device for military applications. Other state of the art analog correlators such as the optical correlators or bulk acoustic wave correlators are even less amenable to inexpensive planar large scale integrated (LSI) circuit fabrication.
Three types of digital cross-correlators are known in the prior art. In the first, a single multiplier and adder are used to accumulate the cross-correlation value. The speed of such a cross-correlator is much slower than that of the required multiplier and adder. In the second type of digital cross-correlators, Fast Fourier Transform (FFT) is used to compute the cross-correlation. If a single multiplier is used in the FFT, the number of multiplication times required is proportional to N log.sub.2 N, where N is the data block length, so once again the correlation speed is slow compared to the multiplier speed. Finally, LSI binary-versus-binary cross-correlators with analog summation have been built. The primary limitation of the latter correlators is their relatively high power dissipation, about 2.5 watts for a length 64 binary-versus-binary cross-correlation. A 10-bit versus 10-bit correlation of length 64 using such modules would require 250 watts and would preclude its use in many applications because of cooling as well as power requirements.
Some of the material herein disclosed has appeared in an article entitled "Improving the Accuracy of Analog Signal Processing Devices by Implementing Residue Class Arithmetic", by James W. Bond, which appeared in Journee's d' Electronique 1975, Advanced Signal Processing Technology.